At the time the Germans first started experiments with magnetic mines in 1923, they realized that they would never have a sufficient supply of nickel to make induction mines. Therefore, all energy was devoted to the development of dip-needle type mine units. They reasoned that, althoug there was nickel available, mining would take secondary pri-ority with respect to other weapons and the aircraft industry. This is undoubtedly the reason why only needle-type mines were laid operationally in any large number. However, when the Germans recovered the first British induction mines, they were stimulated to experimentation with similar circuits and coils. Although it appears that some 200 experi-mental BMA mines were laid, the nickel shortage prevented a serious induction mining effort. At some further advanced points in the war they actually intended to use British coil-rods and other British components recovered from mines dropped on land to lay against the British. This, however, never took place. There is no documentary informa-tion on induction mines; however, samples of the BMA I, II, and III and J-I were found.

Because of the shortage of nickel, cobalt, and copper, several schemes of substitu-tion were used. The shortage of nickel seriously impeded the production of coil-rods, and Krupp was given the job of producing coil-rods of high-premeability transformer steel, which were to be wound with aluminum windings. To overcome the loss of premeability due to nickel shortage, there was to be more than one coil-rod, to make up the loss in sensitivity. These coils were to be arranged in a circle, parallel to the axis of the mine and cast into the explosive. The cylindrical portion of the BMA case into which they were placed was to be of non-magnetic steel. Most BMA mine-cases found were fitted with tubes for two coil-rods. However, others have been found with six and some with eight. Approximately 200 such cases were manufactured, most of which were used for experi-ments. As a result of these unsuccessful experiments, some work was done on "ampli-fied-induction" units, which were to be fitted to the aluminum-cased LMB mine. These experiments were not pursued exhaustively, and no satisfactory unit resulted. The cir-cuits described below are entirely SVK developments. The Luftwaffe was more favored by the German High Command and was allocated a small amount of nickel for induction-mine production. Two induction combination units were completely developed and ready for production at the end of the war in Europe. These were the AJ (D) 102, developed by Dr. Ing. Rudolf Hell, and the JDA 105, developed by Dr. Werner of A.E.G.

Figure 262 – BMA Cross Section